Download Casein and whey exert different effects on plasma amino acid

British Journal of Nutrition (2003), 89, 239–248
q The Authors 2003
DOI: 10.1079/BJN2002760
Casein and whey exert different effects on plasma amino acid profiles,
gastrointestinal hormone secretion and appetite
W. L. Hall*, D. J. Millward, S. J. Long and L. M. Morgan
Centre for Nutrition and Food Safety, School of Biomedical and Life Sciences, University of Surrey, Guildford,
Surrey GU2 7XH, UK
(Received 4 April 2002 – Revised 11 July 2002 – Accepted 6 September 2002)
Protein, generally agreed to be the most satiating macronutrient, may differ in its effects on
appetite depending on the protein source and variation in digestion and absorption. We investigated the effects of two milk protein types, casein and whey, on food intake and subjective
ratings of hunger and fullness, and on postprandial metabolite and gastrointestinal hormone
responses. Two studies were undertaken. The first study showed that energy intake from a
buffet meal ad libitum was significantly less 90 min after a 1700 kJ liquid preload containing
48 g whey, compared with an equivalent casein preload (P,0·05). In the second study, the
same whey preload led to a 28 % increase in postprandial plasma amino acid concentrations
over 3 h compared with casein (incremental area under the curve (iAUC), P, 0·05). Plasma
cholecystokinin (CCK) was increased by 60 % (iAUC, P, 0·005), glucagon-like peptide
(GLP)-1 by 65 % (iAUC, P, 0·05) and glucose-dependent insulinotropic polypeptide by
36 % (iAUC, P, 0·01) following the whey preload compared with the casein. Gastric emptying
was influenced by protein type as evidenced by differing plasma paracetamol profiles with the
two preloads. Greater subjective satiety followed the whey test meal (P,0·05). These results
implicate post-absorptive increases in plasma amino acids together with both CCK and GLP1 as potential mediators of the increased satiety response to whey and emphasise the importance
of considering the impact of protein type on the appetite response to a mixed meal.
Dietary protein: Satiety: Cholecystokinin: Glucagon-like peptide-1: Postprandial amino
acids
It is well established that protein is more satiating, kJ
for kJ, than carbohydrate or fat (Booth et al. 1970;
De Castro, 1987; Barkeling et al. 1990; Hill & Blundell,
1990; Johnstone et al. 1996; Stubbs et al. 1996; Vandewater & Vickers, 1996; Porrini et al. 1997; Latner &
Schwartz, 1999). However, the satiating influence of protein is variable. One influence is the habitual protein
intake that affects the satiating response to a standard protein meal in individuals. Thus, the satiating response to a
standard protein meal is less in subjects previously adapted
to a high- compared with a low-protein intake (Long et al.
2000). While the mechanism for this has not been demonstrated, one possibility relates to a more rapid postprandial
clearance and oxidation of amino acids (AA) in subjects
adapted to higher protein intakes. Another influence is
the protein itself. There is some evidence that different protein sources may differ in their satiating capacity (Uhe et al.
1992), although not all observers report differences (Lang
et al. 1998, 1999). If the extent of postprandial excursions
in circulating AA levels influence satiety (the basis of
Mellinkoff’s original aminostatic concept (Mellinkoff
et al. 1956)), then factors that influence dietary protein
digestion and absorption could influence the relative satiating influence of proteins. The digestion and absorption of
whey and casein differ in that casein, unlike whey, coagulates in the stomach due to its precipitation by gastric acid
(Billeaud et al. 1990; Miller et al. 1990). As a result, overall gastric emptying time for casein is longer and there is a
smaller postprandial increase in plasma AA compared with
the non-coagulating whey protein. The concept of ‘fast’ and
‘slow’ proteins was introduced by Boirie et al. (1997) to
describe these differences in digestion and absorption of
whey and casein. It might be predicted therefore that
whey, a ‘fast’ protein, would be more satiating than casein.
Abbreviations: AA, amino acid; CCK, cholecystokinin; GIP, glucose-dependent insulinotropic polypeptide; GLP, glucagon-like peptide; VAS, visual
analogue scale.
* Corresponding author: Dr W. L. Hall, fax +44 1483 576978, email [email protected]
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240
W. L. Hall et al.
The aims of the present study were therefore: (1) to test
the hypothesis described earlier; (2) to investigate possible
physiological and metabolic bases for any differences in
appetite found. The effects on appetite of isoenergetic
liquid preloads containing either casein or whey were
investigated in normal-weight healthy volunteers, by
measuring their relative influence on food intake and subjective appetite ratings. In a separate experiment, postprandial profiles of circulating hormones and metabolites were
measured following consumption of the two protein preloads. In particular, the potential roles of cholecystokinin
(CCK) and glucagon-like peptide (GLP)-1, two gastrointestinal hormones that have both been previously shown
to play an important role in satiety in human subjects (Ballinger & Clark, 1994; Lieverse et al. 1994; Gutzwiller et al.
1999; Verdich et al. 2001), were investigated.
Materials and methods
Two studies were performed both involving a randomised,
single-blind, within-subject experimental design. In the
first, investigations were limited to study of the appetite
responses to casein and whey preloads by means of subjective ratings and measurement of food intake ad libitum
during a buffet-style meal. In the second study, the focus
was on the physiological and biochemical sequelae of the
two protein pre-meals and a subsequent standard meal,
although subjective appetite ratings were also measured.
Subjects
All subjects were non-dieting and non-smokers. No subjects had special dietary requirements, significant current
or previous medical history, or took medication apart
from oral contraceptives. Written informed consent was
obtained from all participants and the study protocol was
approved by the University of Surrey Advisory Committee
on Ethics.
Subjects in study 1 consisted of sixteen lean healthy volunteers (eight female and eight male) who were recruited
from the University of Surrey, mean age 22 (SD 2·0)
years and mean BMI 21·7 (SD 2·0) kg/m2. Subjects in
study 2 were nine lean healthy volunteers (eight female
and one male) recruited from the University of Surrey,
mean age 25 (SD 2·4) years and mean BMI 22·6 (SD 1·5)
kg/m2. In each case, potential volunteers were excluded
if they consumed . 20 % energy intake as protein (assessed
by 4 d food diary in study 1, and 7 d food diary in study 2)
or if their mean score was . 3·0 for the Dutch Eating Behaviour Questionnaire (van Strien et al. 1986).
Preloads, meals and measurement of appetite
Casein and whey liquid preloads. Identical isoenergetic
high-protein liquid preloads, containing about 1700 kJ
and 48 g protein, were used in both studies. The liquid preload was formulated using commercially available powdered whey (Prolab Nutrition, Bloomfield, CT, USA) or
casein (Tropicana World Ltd, Birmingham, UK), protein,
double cream and maltodextrin made up to 450 ml with
water. Macronutrient and AA compositions of the two
Table 1. Composition of whey and casein protein powder
Energy (kJ/kg)
Protein (g/kg)
Carbohydrate (g/kg)
Fat (g/kg)
Amino acids (g/kg)
Alanine
Arginine
Aspartic acid
Cysteine
Glutamic acid
Glycine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Proline
Serine
Threonine
Tryptophan
Tyrosine
Valine
Whey
Casein
17 270
762
87
48
14 980
850
12
15
48
23
102
12
172
20
16
84
105
91
16
31
61
52
62
21
24
60
31
38
73
4
223
19
32
58
101
83
30
54
105
63
46
14
58
74
protein powders are shown in Table 1 and the macronutrient composition and energy content of the final formulated
preloads in Table 2. In addition, in study 2, the liquid preloads also contained 1·5 g paracetamol (Panadol; GlaxoSmithKline, Brentford, Essex, UK) in order to monitor
gastric emptying (Heading et al. 1973).
Meals and appetite measurement. Food intake after
consumption of the preload was assessed in study 1 using
a buffet test meal ad libitum (Hill et al. 1995) that enabled
subjects to choose from a range of familiar foods, appropriate to the time of day. Subjects were asked to rank various
buffet food items in order of preference. They were then
offered their second and third choices of sandwich fillings
and food types. The total energy available from the buffet
ranged from 10 751 kJ to 12 611 kJ (protein 13, fat 38,
carbohydrate 49 % energy) depending on subjects’ preferences and the subsequent food items offered. The number
and range of food items presented to each subject was identical on each test occasion. The buffet food was weighed
before and re-weighed after each subject had finished
eating, to allow calculation of energy and macronutrient
intake.
A cold lunch was supplied in study 2 based on the subjects’ food choices (obtained during recruitment). This was
standardised to supply each subject with 42 kJ/kg body
weight, and with each component of the meal making up
the same proportion of total energy intake for each individual (calculated on the basis of the nutritional composition
of the foods). The meal choices were similar to those
offered to subjects in the buffet-style meal used in study
1. Subjects were instructed to consume all the food that
was offered.
Subjective assessment of hunger, satiety and desire to
eat was made in both studies using subjective visual
analogue scales (VAS) (Hill et al. 1995). Each scale consisted of a 100 mm line anchored at either end with
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Responses to casein and whey
241
Table 2. Composition of liquid test meals
Casein*
Test meal. . .
Protein powder
Double cream
Maltodextrin
Whey†
g
Macronutrient (g)
Energy (kJ)
g
Macronutrient (g)
Energy (kJ)
57
23
24
48‡
11§
24k
858
414
402
63
19
20
48‡
9§
20k
1021
339
335
* Total energy 1674 kJ.
† Total energy 1695 kJ.
‡ Protein.
§ Fat.
k Carbohydrate.
extreme statements; for example the scale ‘how hungry do
you feel?’ ranged from ‘not at all hungry’ to ‘as hungry as I
have ever felt’. Subjects were instructed to rate themselves
by marking the scale at the point that was most appropriate
to their feeling at that time. Ratings on the scales were converted to a score (mm) for statistical analysis of the appetite responses to each treatment.
Study protocols
Each study was a randomised, single-blind, within-subject
experimental design involving the testing of either whey or
casein liquid preloads on two separate days. Subjects were
instructed to avoid alcohol and high-protein meals the evening before and until the study, and in study 2 to refrain
from taking any paracetamol tablets or paracetamol-containing products for the previous 24 h.
Study 1. Subjects were instructed to consume their
usual breakfast before 09.30 hours, and to eat the same
cereal-based breakfast on each test occasion. Individuals
were offered a liquid casein or whey preload at 11.30
hours on separate occasions 7 d apart. This was presented
in a black opaque beaker with a lid and ingested through
a straw, thus masking any visible difference in the appearance of the preloads. VAS ratings were completed before
and after consumption of the drink, and at 20 min intervals
for the next 3 h. Ninety minutes after the liquid preload,
subjects were offered the pre-weighed buffet-style test
meal ad libitum, from which energy and macronutrient
intakes were calculated.
Study 2. The protocol for the investigation of physiological mechanisms was designed to emulate the procedures in study 1 as closely as possible. On the day of
each study, subjects consumed a standard breakfast consisting of cornflakes and skimmed milk before 08.00 hours,
and then consumed nothing until the study in order that
all volunteers started the study in a similar metabolic
state. Subjects consumed the liquid casein or whey preload
at 11.00 hours. Venous blood samples were taken before,
and at 5, 15, 35, 45, 60, 75, 90, 120 and 180 min after preload consumption from a cannulated forearm vein, for the
measurement of plasma paracetamol, AA, glucose, insulin,
GLP-1, glucose-dependent insulinotropic polypeptide
(GIP) and CCK. The standard lunch was provided 90 min
after the liquid preload. Subjects were instructed to eat
all that was provided as quickly as was comfortable and
blood sampling was continued for another 90 min following lunch.
Biochemical analyses
Blood samples were taken at regular intervals into tubes
containing preservatives. They were kept on ice before
centrifugation at 48C for 10 min at 1550 g to separate the
plasma. Blood was collected into fluoride oxalate tubes
before centrifugation for glucose measurement, and into
lithium heparin tubes for AA, insulin and GIP measurement. Blood was collected into lithium heparin tubes
containing aprotinin (200 kIU/ml blood) for GLP-1
measurement. The separated plasma was frozen at a temperature of 2 208C. For CCK measurement, blood was collected into chilled 5 ml EDTA tubes containing sodium
acetate buffer (1 ml sodium acetate buffer (175 mM -NaCl,
63 mM -NaOH, 30 mM -acetic acid, adjusted to pH 3·6 with
HCl)/4 ml blood). The separated plasma was added to ethanol (plasma –ethanol (980 g/l) 1:2, v/v) on the study day,
vortexed, then centrifuged at 1550 g for 15 min. The supernatant fraction was transferred into assay tubes, immediately dried under low vacuum and the tubes stored at
2 808C until assay. Plasma paracetamol and glucose were
measured using commercial assay kits (Cambridge Life
Sciences (Ely, Cambridge, UK) and Randox (Crumlin,
Co. Antrim, UK) respectively) with an automated Cobas
Mira biochemical analyser (Roche Products Ltd.,
Welwyn Garden City, Hertfordshire, UK). Plasma AA
were measured using Waters PICO-TAGTM Amino Acid
Analysis System (Waters, MA, USA), which involves the
pre-column derivatisation of the sample with phenylisothiocyanate followed by reversed-phase HPLC. Plasma
insulin, GIP, GLP-1 and CCK were analysed using radioimmunoassay techniques (Morgan et al. 1978; Beardshall
et al. 1992; Elliott et al. 1993; Hampton & Whithey,
1993). Inter-assay CV for metabolites were , 15 % for
insulin, GIP, GLP-1 and CCK. Intra-assay CV were smaller. All samples from an individual subject were analysed
within the same assay.
Statistical analysis
Food intake ad libitum from the buffet meal was analysed
using Student’s paired t test. VAS ratings and biochemical measurements were analysed by repeated-measures
analysis of covariance with type of protein and time as
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242
W. L. Hall et al.
within-subjects factors. Baseline values were treated as covariates or, for plasma AA concentrations, an ANOVA was
carried out on the change from baseline values (x - baseline
value). All results are expressed as mean values with their
standard errors unless otherwise stated. P values , 0·05
were considered statistically significant.
Results
Study 1
Energy intake ad libitum for the buffet meal is shown in
Fig. 1, including the proportion of energy consumed as
carbohydrate, fat and protein. Total energy intake was significantly lower following the whey protein than following
the casein protein preload (3676 (SEM 359) v. 4537 (SEM
528) kJ respectively, P, 0·05). This difference was
reflected in intakes of all three macronutrients. Subjective
appetite ratings (VAS ratings) for hunger, desire to eat
and fullness varied significantly over time (P, 0·001).
However, no significant differences were found due to protein type (results not shown).
Study 2
Appetite. VAS ratings for hunger, desire to eat and fullness varied significantly with time (P, 0·001). Desire to
eat (Fig. 2) was significantly reduced by whey protein compared with casein following the standard lunch (P, 0·005),
an effect that failed to reach significance for hunger ratings
(P¼ 0·061). Fullness (Fig. 3) was significantly increased
following the liquid whey preload compared with casein
for the duration of the 180 min (P, 0·05).
Plasma metabolites. AA profiles were measured for
thirteen AA. Total AA profiles are shown in Fig. 4.
There was a significant preload £ time interaction for
plasma total AA (P, 0·005) due to higher circulating AA
Fig. 1. Study 1: effects of 1700 kJ preloads (48 g casein or whey)
on energy intake (including the proportions of total energy intake
from protein, fat and carbohydrate) from a buffet lunch ad libitum
B, Carbohydrate; A, fat; A
B, protein. For details of
90 min later. A
liquid test meals, subjects and procedures, see Tables 1 and 2 and
p. 240. Values are means for sixteen subjects with their standard
errors shown by vertical bars. There was a significant reduction in
energy intake following the whey compared with the casein preload
(P,0·05).
concentrations during the second and third hours following
the liquid whey preload compared with casein. Increases in
postprandial plasma profiles of the branched-chain AA following the whey compared with the casein preload were
particularly marked and highly significant. Increases (%)
following whey compared with casein over 180 min
were: valine 34 (P, 0·001), isoleucine 102 (P, 0·00005)
and leucine 70 (P, 0·0000001), and threonine was
increased by 95 (P, 0·00005). No significant difference
was observed in plasma glucose response following the
two different protein liquid preloads.
Plasma hormones. There were significantly greater
increases in overall plasma concentrations of GIP
(P, 0·005), GLP-1 (P, 0·05) and CCK (P, 0·01) following the whey preload compared with casein (Figs 5, 6
and 7), but postprandial increases in plasma insulin did
not differ in response to the two protein preloads.
Gastric emptying. Plasma paracetamol concentrations
following casein and whey preloads demonstrated a significant preload £ time effect (P, 0·05). This reflects a
difference in rates of gastric empting between the two preloads, with a faster initial rise in plasma paracetamol concentrations following the casein meal and then a levelling
off compared with the whey preload.
Discussion
The present study extends our interest in aminostatic mechanisms of appetite regulation (Long et al. 2000) to an
examination of the differing satiating capacity of various
protein sources (Uhe et al. 1992; Lang et al. 1998,
1999). Here we focus on the ‘fast’ and ‘slow’ protein concept introduced by Boirie et al. (1997) to describe the
differences between whey and casein in their digestion
and absorption. Our present results confirm that casein,
as a coagulating protein, exhibits a slower rate of gastric
emptying and mediates lower postprandial excursions of
plasma AA concentrations compared with the non-coagulating protein whey (Boirie et al. 1997). On the basis of
Mellinkoff’s original aminostatic concept (Mellinkoff
et al. 1956), a larger rise in plasma AA would increase
satiety, and this is demonstrated by our present findings.
Our experimental design involved an initial study of
appetite responses alone (study 1) followed by attempts
to identify physiological and biochemical responses
(study 2). We assumed that the serial blood sampling
might make it more difficult to identify appetite responses,
so we made no attempt to measure food intake ad libitum
in study 2, limiting our measures to observations of subjective differences in appetite (VAS scores). The relative
palatability of the casein and whey preloads was not
measured by VAS or any other methodology. However,
the presentation of the two meals in black opaque beakers
with lids and straws minimised the visual and olfactory
stimulus, and the protein powder formulations used were
both fruit-flavoured, so there was no marked difference
between the sensory qualities of the preloads. If there
was a difference in palatability, the buffet-style ad libitum
test meal in study 1 was 90 min after ingestion, and any
influences on satiety resulting from the palatability of the
preloads would have been minimal by then. In study 2,
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Responses to casein and whey
Fig. 2. Study 2: effects of 1700 kJ preloads (48 g casein or whey) on desire to eat ratings (visual
analogue scale). X, Casein; A, whey. The liquid test meal was given at 0 min and the standard lunch
at 90 min. For details of liquid test meals, subjects and procedures, see Tables 1 and 2 and p. 240.
Values are means for nine subjects with their standard errors shown by vertical bars. There was a
significant reduction in desire to eat after the whey preload following the standard lunch (analysis of
covariance 90– 180 min, P,0·005).
Fig. 3. Study 2: effects of 1700 kJ preloads (48 g casein or whey) on fullness ratings (visual analogue scale). X, Casein; A, whey. The liquid test meal was given at 0 min and the standard lunch at
90 min. For details of liquid test meals, subjects and procedures, see Tables 1 and 2 and p. 240.
Values are means for nine subjects with their standard errors shown by vertical bars. There was a
significant increase in fullness after the whey preload (analysis of covariance 0– 180 min, P,0·05).
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243
244
W. L. Hall et al.
Fig. 4. Study 2: effects of 1700 kJ preloads (48 g casein or whey) on plasma total amino acids. X,
Casein; A, whey. The liquid test meal was given at 0 min and the standard lunch at 90 min. For
details of liquid test meals, subjects and procedures, see Tables 1 and 2 and p. 240. Values are
means for nine subjects with their standard errors shown by vertical bars. There was a significant
increase in total amino acid concentration following the whey preload (ANOVA 1– 180 min,
treatment £ time interaction, P,0·005).
Fig. 5. Study 2: effects of 1700 kJ preloads (48 g casein or whey) on glucose-dependent insulinotropic polypeptide (GIP). X, Casein; A, whey. For details of liquid test meals, subjects and procedures,
see Tables 1 and 2 and p. 240. Values are means for nine subjects with their standard errors shown
by vertical bars. There was a significant increase in plasma GIP after the whey preload (analysis of
covariance 0–180 min, P,0·005).
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Responses to casein and whey
245
Fig. 6. Study 2: effects of 1700 kJ preloads (48 g casein or whey) on plasma glucagon-like peptide
(GLP)-1. X, Casein; A, whey. For details of liquid test meals, subjects and procedures, see Tables
1 and 2 and p. 240. Values are means for nine subjects with their standard errors shown by vertical
bars. There was a significant increase in plasma GLP-1 after the whey preload (analysis of covariance 0– 180 min, P,0·05).
the largest differences in subjective appetite ratings were
also observed following the standardised lunch at 90 min.
The 19 % reduction in food intake ad libitum following a
liquid whey preload compared with the casein in study 1
clearly demonstrated that a whey-based liquid meal has a
greater satiating efficiency than a casein-based liquid
meal. Although no differences in subjective measures of
appetite were shown in study 1, in study 2 significant
Fig. 7. Study 2: effects of 1700 kJ preloads (48 g casein or whey) on cholecystokinase (CCK).
X, Casein; A, whey. The liquid test meal was given at 0 min and the standard lunch at 90 min. For
details of liquid test meals, subjects and procedures, see Tables 1 and 2 and p. 240. Values are
means for nine subjects with their standard errors shown by vertical bars. There was a significant
increase in plasma CCK after the whey preload (analysis of covariance 0–180 min, P,0·01).
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W. L. Hall et al.
differences were observed. Thus, the whey preload resulted
in increased fullness for the duration of the study and suppression of both hunger and desire to eat each following
the standardised lunch-type meal. We interpret our
inability to detect differences in the VAS scores in study
1 as reflecting the difficulty of quantifying this measure
of appetite (Rogers, 1993; Hill et al. 1995; Green & Blundell, 1996; Flint et al. 2000; Stubbs et al. 2000). While all
subjects appeared motivated and compliant with the experimental procedures, the subject groups in the two studies
did differ to some extent. Subjects in study 2 were slightly
but significantly older (mean age 25 v. 22 years, P, 0·037).
In study 2, unlike study 1, subjects were compensated
financially for their time and the cannulation procedures.
We do not know whether these factors influenced the subjects’ use of the VAS ratings. However, the largest differences in subjective appetite ratings found in study 2 were
recorded following the standardised lunch. These differences are likely to have been obscured in study 1 since subjects were allowed to eat ad libitum and therefore ate
different amounts at each study session. The differences
in hunger and desire to eat were mainly evident following
the standard lunch, whereas subjective ratings of fullness
were different throughout the 180 min. This observation
reflects the fact that ‘hunger’ and ‘satiety’ are distinct sensations and should not be regarded as simply the two
extremes of a continuum. For example, the initiation of
an eating episode does not wholly rely on hunger sensations, since the sensory properties of a food can stimulate
food intake even when satiety signals are present. The fact
that the protein preloads were administered as a covert
liquid meal rather than a more customary solid meal may
have meant that the cognitive and sensory stimulus that
normally inhibits desire to eat was repressed until the consumption of a more familiar solid meal, the standard lunch.
Sensations of fullness, however, are strongly determined
by sensations of gastric distension, and this may account
for the apparently consistent difference in satiation
between the two preloads throughout the whole 180 min
of the study.
In the experiments presented here, a difference in food
intake or subjective satiety ratings was observed during a
set period of 180 min following the casein and whey preloads. It is not clear from these present studies however,
whether these differences would persist beyond that time
period, or in fact whether the ‘fast’ protein would prove
to be less satiating in the longer term than the ‘slow’ protein. Subjective fullness ratings and plasma CCK concentrations were still significantly increased following the
whey at 180 min (analysis of covariance, P, 0·01 and
P, 0·05 respectively), which could potentially impact
satiety at the next meal. This suggests that the satiating
capacity of whey could be maintained over a longer
time-period than included in the current work. Longerterm studies are needed to address the question of whether
whey protein is more satiating than casein over a day, or
whether there would be a compensatory increase in food
intake at the next meal, in which case there may be no
difference in total food intake.
As judged by the plasma paracetamol profiles, an index
of gastric emptying, differences in the gastric emptying
profiles of the liquid casein and whey preloads occurred,
although the nature of these differences was complex.
Initial paracetamol absorption was faster following
casein, but peak circulating concentrations were higher
with whey. The most likely explanation of this pattern is
that after the casein protein had precipitated into curds in
the stomach, the solubilised paracetamol in the liquid compartment of the meal was emptied and absorbed rapidly,
but the slow emptying of the solid casein curds maintained
paracetamol absorption with a plateau in plasma concentrations. Indeed, it is possible that the whey meal emptied
homogeneously at a steady rate, with a more even dispersion of the lipid and aqueous phases containing the
carbohydrate and protein in the whey liquid meal, whereas
the casein meal emptied in a biphasic manner due to its
transformation to a solid – liquid meal, with the liquid component emptying earlier. Despite the apparent differences
in gastric emptying rates of the two preloads, the time
course of plasma glucose and insulin concentrations were
not significantly different, suggesting that carbohydrate
emptied at a near uniform rate with the liquid component
of the casein meal. This is in contrast to previous reports
of differences in plasma insulin levels following different
meal proteins (Gannon et al. 1988; Hubbard et al. 1989;
Lang et al. 1999). Both insulin and glucose have been
implicated in the control of appetite (Woods & Porte,
1983; Campfield & Smith, 1990); however, in the present
study, it is unlikely that the observed differences in appetite could have been mediated by either glucose or insulin
levels. This identifies variation in either protein- or fatmediated mechanisms of satiety as responsible for the
satiating effect of whey.
The marked differences in circulating postprandial AA
levels observed following the two protein preloads point
to some aminostatic mechanism of appetite regulation as
a likely explanation of the differences in appetite observed
with the two proteins. Postprandial peripheral AA profiles
are likely to reflect rates of digestion, absorption and first
pass metabolism in the enterocyte and liver in addition to
the AA composition of the proteins. The greatest differences between whey and casein AA composition (g/kg)
were for arginine, methionine, phenylalanine, proline and
tyrosine, in each case . 50 % more in casein compared
with whey. Just under half of the plasma AA measured
(five out of thirteen) showed an overall greater plasma
response to the whey preload. The remainder showed no
significant differences in their profiles throughout the
study with the exception of proline, which significantly
increased following the casein meal, probably reflecting
the higher proline content of casein protein. Phenylalanine,
whose effects on satiety are thought to be potent (Gibbs &
Smith, 1977; Muurahainen et al. 1988; Ballinger & Clark,
1994; Rogers et al. 1995), showed no significant differences in plasma levels following the two test meals,
again probably due to the greater phenylalanine content
of casein. Tryptophan, an AA frequently linked with appetite regulation (Wurtman et al. 1981; Hrboticky et al. 1985;
Hill & Blundell, 1988), could not be measured using our
methodology.
The gastrointestinal hormones CCK and GLP-1 are
secreted in response to protein ingestion (Hopman et al.
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Responses to casein and whey
1985; Elliott et al. 1993). CCK is stimulated by the entry of
protein into the duodenum (Go et al. 1970), and GLP-1
secretion is influenced by the rate of gastric emptying
(Schirra et al. 1996). Both hormones have previously
both been shown to play an important role in satiety in
human subjects (Ballinger & Clark, 1994; Lieverse et al.
1994; Gutzwiller et al. 1999; Verdich et al. 2001). In the
current study, we have investigated the potential role of
these gastrointestinal hormones in mediating differences
in appetite following ingestion of casein or whey. Both
CCK and GLP-1 responses are consistent with a role in
the greater satiating influence of whey, since both were
increased more following the whey rather than casein preload. We hypothesised that if the whey protein preload was
emptied from the stomach more rapidly than the casein
protein, the protein-induced CCK and GLP-1 responses
would be larger following the whey preload, and this
indeed appeared to be the case. However, the degree to
which the higher plasma CCK and GLP-1 concentrations
were a result of the fat component or the protein component of the whey meal was not clear. If it is assumed
that the mixture emptying into the duodenum during the
90 min pre-standard lunch study period contained a greater
proportion of protein per ml following the whey preload
compared with the casein, then it is likely that the higher
CCK and GLP-1 levels in the plasma were due to the
greater secretagogue effect of whey protein. However,
secretion of the gastrointestinal hormone GIP was also
greater following the whey preload. GIP secretion is not
stimulated by protein (Elliott et al. 1993); on an isoenergetic basis, fat is the most potent stimulator of GIP
secretion in human subjects (Penman et al. 1981). GIP
secretion is a marker for carbohydrate and fat absorption
(Morgan et al. 1988), suggesting that a greater proportion
of fat, in addition to protein, was being emptied and
absorbed immediately following the whey preload. This
may have been a result of incorporation of fat into the
casein curds or possibly a generally stronger inhibition of
gastric emptying through a neural reflex response to the
retention of solid matter in the fundus.
In conclusion, a preload drink of whey protein has been
shown to be more satiating, and to be associated with
higher postprandial circulating levels of AA, CCK and
GLP-1, compared with an isoenergetic casein preload.
These findings identify larger post-absorptive increases in
plasma AA together with both CCK-mediated and GLP1-mediated satiety mechanisms as potential mediators of
the increased satiety response to whey. Further work will
be needed to identify the relative importance of these
responses. It is difficult to separate these hormonal and
metabolite responses, since they will always occur together
in a natural context, and in reality their influences on satiety probably complement each other. Regardless of the
physiological mechanisms for increased satiety following
whey protein compared with casein, these findings raise
the opportunity for research into the development of new
food products that differ in their satiating capacity. These
could include foods containing ‘fast’ proteins that would
increase the satiating capacity of the food, thus helping
in the treatment of obesity and weight management, as
well as less satiating foods containing ‘slow’ proteins
247
that might be useful in clinical nutrition as sip feeds for
elderly or sick patients where increased food intake is
desired.
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